1. Field of the Invention
The present invention relates to a voltage sensing circuit for sensing, for example, the power supply voltage or a boosted voltage in a semiconductor integrated circuit.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-311643 describes a voltage sensing circuit comprising a bandgap current generator, a differential amplifier, and a voltage comparator. The bandgap current generator outputs a constant current with substantially no temperature dependence. The differential amplifier amplifies a reference voltage obtained by passing the constant current through a resistor. The voltage comparator compares the amplified reference voltage with the voltage to be sensed.
This voltage sensing circuit is extremely stable, being highly insensitive to temperature variations, but it is not entirely free of problems. One problem is that to generate the constant current, the bandgap current generator requires at least a certain minimum supply voltage level. If the power supply voltage is below this minimum level, the bandgap current generator cannot operate properly and the necessary temperature-independent reference voltage cannot be obtained. Another problem is that the differential amplifier that amplifies the reference voltage generally operates with a temperature-dependent bias current. A further problem is that the voltage level comparator (another differential amplifier) does not operate properly unless the voltages compared (the amplified reference voltage and the voltage to be sensed) have at least a certain minimum level. Consequently, the voltage sensing operation is not completely temperature-independent, and produces erratic results when the power supply voltage is too low.
The present invention addresses these problems.